Earth even with massive temperature swings from today (due to wacky orbit or whatever) I think Earth would still be more hospitable than Mars. It has a magnetic field and a moon and lots of water. If all the life has died, and the atmosphere has frozen or boiled off, well, you're still no worse off than Mars, and you've still got that magnetic field and aquifers and petrochemicals to get back to space eventually.

Earth has active plate tectonics, volcanism, hurricanes/tornadoes (with enough mass in them to destroy things), land slides, floods, lightning, and maybe worst of all other humans.

If we're living in a closed-ish habitat anyways, I think Mars might even manage to be easier (provided there was a critical mass of humans living on it). It seems like the only unpredictable natural disasters there are dust storms. Especially if you consider that any post apocalyptic habitat scenario is going to leave a lot of people out, and they aren't going to be peaceful about that during the transition.